There are many applications in joinery in which two members must be secured together by fasteners in such a way that the fastening elements are not visible from the outside surfaces of the resulting structure. In U.S. Pat. No. 3,496,974, for example, a stepped drill bit is disclosed which is suitable for forming a counterbore and fastener receiving bore in the back side of a wooden member proximate the edge thereof to permit the formation of a face frame. The counterbore and the screw receiving bore are both formed at a relatively shallow angle (or oblique angle if measured from the drill axis in the direction of advancement of the drill) with respect to the back surface of the member being drilled proximate an edge thereof. A screw positioned in the screw receiving bore hole and extends outwardly of the edge of the member to permit threading of the screw into a second member. As shown in U.S. Pat. No. 3,496,974, the members are oriented in substantially the same plane and joined together in abutting relation. It is also possible to use this same screw pocket fastening approach to join one member in a perpendicular orientation to another member.
U.S. Pat. No. 3,675,312 discloses an alternative apparatus and process for joining together two members in perpendicular relation to each other. This approach also employs an obliquely oriented tool access hole and a large counterbore which is drilled into the edge of the member which will carry the screw. An annular insert is then adhesively secured in the counterbore with the screw in the central bore of the insert, and the access opening is used to drive the screw into the second member.
It is very difficult to drill a counterbore at a shallow or oblique angle with respect to a workpiece since the drill will tend to wander. More particularly, however, it will be deflect toward a parallel orientation with the surface as it enters. The bore that is cut is very often ragged and characterized by chipping. For particle board and hardboard, as well as very hard lumber, drilling at a shallow angle, even with jigs or fixtures, is almost impossible. Similarly, for plastics having low friction surfaces it is extremely difficult to start a stepped drill having a configuration as shown in U.S. Pat. No. 3,496,974.
The approach shown in U.S. Pat. No. 3,675,312 has the disadvantage of requiring a separate insert piece which must be secured by a separate process, as well as a specialized tool. Moreover, the time required to form a joint is undesirably long. Other even more complex face-framing apparatus can be seen in U.S. Pat. Nos. 1,335,544 and 1,602,658.
A better approach is shown in U.S. Pat. No. 4,603,719, as shown in FIG. 1, which discloses a router assembly 10 and an opposed drill assembly 11 both rigidly mounted to a common carriage 12 which is either slidably or pivotally reciprocated. This enables sequential engagement of a workpiece 13 by the router assembly and the drill assembly from opposed directions to form a screw pocket 14 shown in FIG. 2.
Because router assembly 10 is formed to progressively engage surface 15 of workpiece 13 at a relatively shallow oblique angle, a large lateral force must be applied on router bit 16 to enable it to accurately and effectively cut the tapered recess or counterbore 17. In the linear sliding carriage embodiment (not shown), this required lateral force is difficult to manually apply.
A more efficient arrangement to manual application of the router assembly is the pivotal carriage embodiment of FIG. 1. The pivot nature of the carriage furnishes the operator much more leverage to more easily apply the necessary lateral force on the router bit 16. This arrangement fixedly mounts router assembly 10 and drill assembly 11 to common carriage 12 which is pivotally mounted to casing 18 about pivotal axis 20. Upon sequential engagement of the router assembly 10 and drill assembly 11 with workpiece 13, screw pocket 14 is formed consisting of counterbore 17 and bore 19 communicating with the counterbore (FIG. 2).
One problem associated with this pivotal configuration is that to assure a relatively linear drilling path of drill bit 21 upon engagement with workpiece 13, the pivotal radius or pivotal arm (illustrated by broken line 22 in FIG. 1) of drill assembly must be relatively long. Too short a pivotal arm will detrimentally cause drill assembly 11 to move along too arcuate a path. This causes the drill bit 21 to engage the workpiece at too steep an angle which reduces the linearity of bore 19. Moreover, substantial lateral forces are induced on the drill bit which results in premature wear or breakage of the drill bit during operation.
However, a long pivotal arm 22 is disadvantageous as well because the overall height dimension of the apparatus must be proportionately tall to accommodate the height and pivotal motion of carriage 12. Since these screw pocket forming apparatuses are often portable in natured table top use is often precluded.
Another problem associated with this arrangement is that the long pivotal arm increases the collective arcuate motion of drill assembly 11 and router assembly 10 which in turn increases the depth dimension of the casing. Accordingly, the overall compactness of the apparatus is compromised, as well as increasing manufacturing costs.